Bovine leukemia is a disease which has serious implications for the cattle industry. Studies on the effect of bovine leukemia have been conducted in the United States and Europe and they support the view that this virus is the infectious etiologic agent responsible for the bovine enzootic leukosis associated with lymphosarcoma.
It is difficult to evaluate the actual incidence of this disease in cattle in the United States. Apparently, there are not as many herds in this country that show as high a percentage of tumors as are found in European cattle, but in 1974, more than 4,500 cattle carcasses were condemned in the United States because of lymphosarcoma.
The bovine leukemia virus belongs to the category of viral agents which are known as oncornaviruses, a name for oncogenic and tumorogenic RNA viruses. This oncornavirus generally does not produce a high percentage of tumors in cattle but it is insidious because it may cause abnormalities other than tumors. Among these are infertility and decreased meat and milk production.
The impact of bovine leukemia on dairy and meat production is difficult to assess in the absence of a reliable serological test for the study of the epidemiology of bovine leukemia.
Another difficulty in studying the oncogenic potential of this virus in cattle is the expense of maintaining them over a long enough period of time so as to allow them to develop tumors. In general, using present techniques, it is necessary to study the animals over a period of 5 to 7 years. This could explain why the incidence of disease in the United States is less than in Europe because, on the average, cattle in this country are slaughtered within about their fourth or fifth year, whereas, many animals in European herds are kept for 10 to 12 years or longer.
Studies have also shown that there appears to be more animals infected in dairy herds than in beef herds. This could very well be due to the management of the various herds. Dairy herds have more contact with pooled colostrum and nurse cows, and more intimate contact with infected animals then do beef animals who generally nurse their own calves and are kept a greater distance from other beef cows.
The bovine leukemia disease appears to be one that is not highly contagious, but it can spread horizontally, that is, from one animal to another in a herd. Bovine leukemia virus can result in a persistent lymphocytosis in some cattle and this condition has been related epidemiologically to a pre-cancerous state. In some countries lymphocytosis is used as a means of detecting and attempting to eradicate bovine leukemia but studies have shown that many cattle develop tumors without developing that condition. However, animals which do not exhibit lymphocytosis are still susceptible to infection and to the oncogenic potential of the virus and, therefore, they can still develop tumors. For this reason, it is generally believed that bovine leukemia canot be eradicated solely on the basis of lymphocyte counts.
It is known that cattle infected with bovine leukemia virus develop persistent infections and most, if not all, of these infected animals develop specific and detectable antibodies. Numerous serological studies have been conducted and they show that the presence of these antibodies are the result of infection with the bovine leukemia virus which produces viral antigens and a correlative antibody response.
Immunodiffusion reaction techniques have been used for the detection of antibodies to an ether resistant antigen of bovine leukemia virus [J. M. Miller, et al.; Journal of the National Cancer Institute, Vol. 49: pages 1459-1462 (1972)].
This technique consists of an immunoprecipitin reaction between an antigen and an antibody in a semi-solid gel medium such as agar. The antigen and antibody are initially soluble in the said medium to the extent that they diffuse or permeate through the matrix material but the reaction product resulting from their combination is insoluble and may be observed visually. The strength of the antigen must be such that it will form a precipitin line of identity with the antiserum in the semi-solid gel medium.
Unfortunately, the bovine leukemia antigen previously used in immunodiffusion tests elicits a very weak antibody response. That antigen, designated by Onuma, et al.,* as an ether-stable molecule (gs-Ag), is an internal antigen of the bovine leukemia virus. Numerous studies with gs-Ag indicate the presence of antibodies but, since the antibody response is very weak, that antigen cannot serve as the basis for a reliable diagnostic test. FNT *M. Onuma, et al.; Journal of the National Cancer Institute, Vol. 55: No. 5, pages 1155-1158 (November, 1975).